In general, in a gas combustion-type driving tool, a driving piston is provided so as to be slidable in an up-down direction within a driving cylinder disposed within a body. A tubular movable sleeve is fitted on the outer peripheral side of the driving cylinder on the cylinder so as to be movable in the up-down direction. The movable sleeve is biased downward by a spring so as to be always located at the lower position. Upon a striking operation, the movable sleeve is moved upward against the spring and abuts to a cylinder head provided at above the driving cylinder to thereby form an air tight combustion chamber. Then, when combustion gas within the combustion chamber is ignited and explosively burnt, a driver is driven together with the driving piston to drive out a fastener.
Conventionally, a spring member for biasing the movable sleeve downward is positioned beneath or on the side surface of the driving cylinder (Patent Documents 1 and 2).
For example, in a gas combustion-type driving tool according to the Patent Document 1, the spring member is provided at the forward position of a nose portion disposed beneath the cylinder to thereby bias a contact arm beneath the gas combustion-type driving tool.
Further, FIG. 2 of the Patent Document 2 shows that a spring member is provided around the driving cylinder within the gas combustion-type driving tool, and FIG. 8 shows that a spring member is disposed beneath the cylinder to thereby bias a contact arm beneath the gas combustion-type driving tool.    Patent Document 1: JP-A-2005-212060    Patent Document 2: JP-B-07-036985
However, in a system where an elastic member such as a spring member is provided beneath a cylinder like the gas combustion-type driving tool shown in FIG. 9 of the Patent Document 2, it is required to provide a dedicated space for disposing the elastic member and so there arise a problem that the entire height of the gas combustion-type driving tool becomes high.
Further, in the case where the spring member is provided on the front surface of the nose portion or around the driving cylinder like the gas combustion-type driving tool described in the patent document 1 or shown in FIG. 2 of the Patent Document 2, a protection cover is required in order for an operator to position a driven member at the portion. Thus, there arises a problem that a size around the nose portion becomes larger and so it is difficult to drive a nail in a narrow space etc.
Further, according to the aforesaid arrangements of the spring member, there arises a problem that the exchanging procedure of the spring member is complicated and troublesome.